There have conventionally been used a simple box-like mixing box as a mixing box for admixing a plurality of gases having different masses. In the case of such a conventional mixing box, the internal volume of the box should be changed depending on the flow rates and kinds of material gases and reactive gases and therefore, it is necessary to use a larger mixing box as the molecular weight ratio of the material gases (raw gases) to the reactive gas increases.
In general, when forming a thin film according to the CVD process such as the MOCVD process, a film-forming gas (a film-depositing gas) comprising vaporized gases and reactive gases is introduced into a vacuum-processing chamber serving as a reactor (a reaction chamber). However, it is necessary to establish the uniform distribution of the flow rate and concentration of the film-forming gas as well as the uniform temperature distribution just above a substrate in order to form a thin film having uniform film quality and uniform film thickness distribution. To establish a uniform film-forming gas stream, the formation of any turbulent flow whose control is quite difficult should be prevented. Thus, the vaporized gas and the reactive gas are uniformly admixed together prior to the introduction of the mixture thereof into the reactor to thus feed the same to the surface of the substrate. For this reason, the vaporized gas and reactive gas are uniformly admixed together in a mixing box equipped with a slit and then introduced into the reactor and fed to the area proximity to the substrate.
Alternatively, there has also been proposed a method comprising the steps of feeding a vaporized gas and a reactive gas to a region proximity to a substrate in a reactor and mixing these gases therein to form a uniform film-forming gas.
Moreover, when feeding material gases using a system for vaporizing liquid materials, the pressure within the vaporization system greatly affects, for instance, the stable vaporization of the liquid (raw) materials, the formation of residue and the service life of the vaporizer and therefore, it has in general been recognized that the pressure within the vaporizer is preferably as low as possible. For this reason, the pressure within the mixing box has conventionally been reduced to a level as low as possible so as to reduce the pressure within the vaporization chamber
In such conventional techniques, however, the following problems arise. If the size of the mixing box increases in the conventional mixing box, the mixing box cannot directly be connected to the reactor from the viewpoint of the structures thereof or the mixing box and the reactor should be kept apart from each other. If they are arranged while they are kept apart from one another, the path for the introduction of the film-forming gas becomes too long and the uniformly admixed film-forming gas would again be converted into a laminar flow during the introduction thereof into the reactor and supply thereof to the region above a substrate in a reactor. For this reason, the mixing box should have a limited size such that it can directly be connected to the reactor and the size of the former cannot be increased at any time.
On the other hand, when increasing the size of a conventional mixing box, a problem arises such that the gas mixture comprising a vaporized gas and a reactive gas becomes a laminar flow when the molecular weight of the vaporized gas is greatly different from that of the reactive gas, in the mixing box having a conventionally known structure, and that it is quite difficult to uniformly admix these gases.
When admixing the vaporized gas (raw gas) and the reactive gas in a mixing box provided with a conventional slit and when the molecular weight of the former is greatly different from that of the latter, the resulting mixed gas may pass through the slit while maintaining the laminar flow condition. Thus, it has been needed to use a complicated mechanism for, for instance, rotating the slit and therefore, there has been desired for the development of a mixing box having a simple structure of construction.
Moreover, in the case of the foregoing method in which a uniform film-forming gas is prepared by admixing vaporized gases (raw material) and reactive gases in the proximity to a substrate, a turbulent flow is formed by the injection of the vaporized gas and it becomes impossible to control the film-forming gas flow comprising the vaporized gases and reactive gases. This accordingly results in a problem such that the thin film formed on the substrate is deteriorated in, for instance, the film quality and the film thickness distribution.
Moreover, when supplying a deposit gas to a reactor using the foregoing system for vaporizing the liquid materials and when using a conventional mixing box, it is quite difficult to design the box in such a manner that an internal pressure thereof can be reduced. For this reason, when the pressure within the mixing box is high, even if the vaporized gas can uniformly be admixed with the reactive gas, the pressure within the vaporizer on its primary side is necessarily high and a problem accordingly arises such that an unstable film-forming gas is introduced into the reactor, a large amount of residue is formed and the maintenance cycle of the vaporization region is shortened.
Incidentally, a CVD system is employed for forming or depositing a desired thin film on a substrate such as a silicon wafer and has a vacuum reactor equipped with a vacuum exhaust means. A stage on which a substrate is placed is arranged in the interior of the vacuum reactor and a gas head is placed at the upper portion of the reactor in such a manner that it is opposite to the stage. A reactive gas and a vaporized gas (raw gas) are fed to a gas-mixing box fitted to this gas head through a gas piping work. When forming a thin film, a mixed gas (film-forming gas) comprising a reactive gas and a vaporized gas mixed together in a desired mixing ratio is introduced into the vacuum reactor, which is maintained at a desired degree of vacuum, through the gas head to induce a gas phase chemical reaction and to thus form a desired thin film on the substrate. In this respect, the exhaust gas containing, for instance, the mixed gas, which does not contribute to the film-forming process, is discharged to the exterior by a vacuum exhaust means.
In this case, to make, the uniform, the distribution of the film thickness and that of the composition of the thin film formed on the substrate and to improve the reproducibility thereof, one should make, uniform, the flow of the mixed gas in the vacuum reactor. In this respect, if a non-uniform flow of the mixed gas is established in the vacuum reactor and a film grows on the inner wall of the vacuum reactor, not only the maintenance of the reactor should frequently be carried out, but also the dust of films thus formed may adversely affect the CVD process.
Thus, in the CVD process in which a mixed gas is introduced into a vacuum reactor while evacuating the same with a vacuum exhaust means, it is necessary to prevent the occurrence of any uncontrollable convection current and turbulent flow in the vacuum reactor. To this end, it is quite important to subject the exhaust gas to isotropic discharge from the peripheral region of the substrate. Moreover, when the CVD process is carried out while applying heat, the occurrence of any heat convection would make the gas flow non-uniform and therefore, one should devise a measure to inhibit the occurrence of any heat convection.
As an example of the means for solving these problems concerning, for instance, the flows of mixed gases and exhaust gases, there has conventionally been proposed a technique in which a fin is fitted to a stage and the stage is rotated during the formation of a film to thus minimize the influence of, for instance, any turbulent flow, convection current and heat convection (see Japanese Un-Examined Patent Publication No. Sho 61-2318). In this case, however, if a system for rotating the stage is placed within the vacuum reactor, a problem arises such that not only the structure of the CVD system becomes complicated, but also the production cost of the film increases. Moreover, in this case, a substrate should be transported to the stage while going round the fin and therefore, a complicated substrate-transporting means should thus be used.
In addition to the foregoing, there has conventionally been known a single wafer-processing type CVD system equipped with a load-lock chamber. According to this system, however, a port for transporting substrates, provided with a gate valve, is positioned on the upper side of the stage and therefore, a problem arises such that the presence of this port for transporting substrates becomes a cause of convection current and/or turbulent flow. In this case, the vacuum reactor may be enlarged to make the influence of such convection current and/or turbulent flow small. However, a new problem arises such that a vacuum pump having a high evacuation ability should be used and that this method is economically unfavorable and runs counter to the recent requirement for the miniaturization of the system.
Accordingly, it is an object of the present invention to solve the problems associated with the foregoing conventional techniques and more particularly to provide a mixing box, which has a simple structure, whose voluminal size is not influenced by the flow rate and kind of the gas to be introduced into the same and which can uniformly admix a plurality of gases having different molecular weights in a high efficiency under a low pressure; a thin film-manufacturing system, which is provided with this mixing box, has a simple structure, permits the deposition of a thin film at a low cost and which can suppress any occurrence of a turbulent flow, a convection flow and heat convection to thus make the flow of the mixed gas uniform; and a method for preparing a thin film.